EXCLUSIÓN DE RELACIÓN LABORAL

The fast neutron sources are necessary both in practical application and in lab- oratory research. As an example of practical application is a non-invasive detec- tion of explosives, drugs and other low atomic number contraband using neutrons [85, 86, 87, 88]. The drugs and explosives typically contain low-atomic number el- ements as H, C, N, 0, P, S, and Cl. An interaction probability of x-rays, which are commonly used for contraband inspections, with low-atomic number elements is low [85]. It leads to development of many neutron-based contraband detection techniques [85, 86, 88]. For example, Thermal neutron analysis (TNA), Fast neutron analysis (FNA), Pulsed fast neuron analysis (PFNA), Pulsed fast neutron transmis- sion spectroscopy (PFNTS), Associated particle imaging (API), Pulsed fast-thermal neutron analysis (PFTNA), Fast neutron scattering analysis (FNSA), etc. [85]. We could found more such methods in [85, 88]. A detailed description of these methods is beyond the scope of this thesis. A possibility of usage of the plasma focus as a neutron source for detection of explosive and illicit materials was reported in [89].

In the similar manner like the contraband detection, the neutron sources are used in geology, planetology, and search for raw materials [90, 91].

The neutron radiation is necessary for many research activities or technological tasks. Obviously, the neutrons are generated in nuclear fission reactors, but usu- ally it is inconvenient or impossible to use the nuclear fission reactor. Therefore, the neutrons are also produced by the laboratory sources. The neutron sources could be based on the spontaneous fission. The common such source is252Cf, how- ever, it is very expensive (hundreds of thousands e [92]) and its half-life is 2.65

Figure 2.7: A system PINS (Portable Isotopic Neutron Spectroscopy) for identifica- tion of munition and chemical weapons used by US army [88].

years only. The widespread sources are also the sources based on (α,n) reactions, typically 241_{Am-Be, or} 239_{Pu-Be. Their half-lifes of 433 years and 24 000 years,}

respectively are significantly longer than the half-life of the 252_{Cf, but their costs}

are also higher. Moreover, it is not possible to simply “switch-off” these radioiso- topic sources and a safe storage and ecological liquidation are complicated. Another possibility is to use sources which produce neutrons by the nuclear reactions of an accelerated particle beam with an appropriate target. For example, often used re- actions are D(d,n)3_He, 9_Be(d,n), 7_{Li(p,n), etc. Such an approach is close to the}

mechanism of neutron production on Z-pinches and plasma foci with deuterium or deuterated liners. The significant difference between the neutron radiation produced by the accelerator neutron source and Z-pinch or plasma foci is that the accelerator source usually produces the neutrons continuously whereas the Z-pinch is inherently a pulsed device. However, in such relatively short neutron pulse, the Z-pinch is able to generate tremendous amount of the D(d,n)3_{He neutrons (3.9 × 10}13 _during

tens of nanoseconds [93]). The short and very intensive neutron fluxes are required for many laboratory purposes. For example, it could be used for the study of the multiple neutron capture reactions which are known as the r-processes. Another laboratory application which requires the intensive neutron pulse is the production of isotopes with a high radioactivity and a short half-life. In such a case, the irra-

diation of a material sample should not be longer than the half-life of the produced isotope, since the decay during the irradiation limits the maximum radioactivity of the sample. The short and intensive neutron pulses generated by Z-pinches allow obtaining the radioisotopes with a practically unlimited half-life (assuming that the neutron pulse duration is on the order of nanoseconds)5_{. In practice, it could be}

used for example in the neutron activation analysis6_{. As far as the long-duration}

neutron production is concerned, we note that some Z-pinch modifications, namely small plasma foci with a peak current up to 100 kA could operate in a repetition regime with a frequency up to 10 Hz and produce the neutron bursts continuously [10].

As an example of the repetitive plasma focus neutron source we could mention the PF-6 device with an energy of 6 kJ developed by The Institute of Plasma Physics and Laser Microfusion (IPPLM) in co–operation with the Moscow Physical Society (MPS) [37]. The PF-6 device produces current pulses with current of about 370 kA and shot repetition rate up to 10 Hz [37]. The neutron yield of the PF-6 device achieves 108 _{neutrons per single shot [37]. Another mobile and high-repetition rate}

plasma focus with similar parameters we found also in ENEA7 in Italy. Even higher shot repetition rate of 100 Hz was achieved on a portable plasma focus developed by Alameda Applied Sciences Corporation (AASC) [43]. Other examples of small high-repetition rate plasma foci neutron sources can be found all over the world [44, 45, 46].

A substantially different group of plasma focus neutron sources are single shot devices. Whereas the high-repetition shot rate plasma foci are often compact and portable devices, the single shot plasma foci are usually larger, they could occupy whole laboratory or a large part of building. Naturally, their electrical parameters and neutron yield per single shot are significantly higher. We note, that the single

5_{The production of radioisotopes with a very short half-life on Z-pinches could be achieved also}

by the interaction of the pulsed ion beams with a material sample.

6_{The neutron activation analysis lies in the activation of a sample and subsequently the ra-}

dioisotopic content is determined by the gamma-ray analysis. The original chemical content is evaluated by the known nuclear reactions, their cross-sections and natural isotopic content of the chemical elements. The advantage of this method is that it is nondestructive. Thus, it is often used for analysis of works of art and historical artifacts.

7_{The Agenzia nazionale per le nuove tecnologie, l’energia e lo sviluppo economico sostenibile}

Target

Plasma focus

(a) (b)

Figure 2.8: (a) Monte Carlo N-Particle transport code (MCNP) simulation of a neutron flux of the neutron-diagnosed subcritical experiment (NDSE) [52]. (b) A photography of the static NDSE target in front of the concrete shielding with ports for the neutron irradiation by the plasma focus and gamma ray detector [52].

shot plasma foci are more common (classical) and most of plasma focus research programs are specialized on the single shot devices. There are various motivations of the single shot plasma focus experiments. Since the mechanism of the neutron production is still not fully clarified, it is studied at many mega ampere-class facilities like PF-1000 [47], PF-360 [48], SPEED 2 [49], FF-1 [50], HAWK DPF [51], etc. A single shot plasma focus device could serve also as an external neutron source for the neutron diagnosed subcritical experiment (NDSE) [52]. The NDSE makes possible to study the nuclear fission under conditions like those at a nuclear explosion, but without the explosion [52]. This topic is very actual due to the international full- scale nuclear test ban treaty [52, 53, 54]. The principle of such NDSE is as follows. A special nuclear material (SNM) target is exposed to the intensive burst of multi- MeV neutrons produced by the plasma focus device. This external neutron burst induce fission chain events in the SNM target and neutron multiplication occurs. At the same time the fission gamma rays are generated. A modern diagnostic techniques make possible to detect both neutrons and gamma rays with nanosecond time resolution [55, 56]. Using time-of-flight and coincidence methods it is possible

to distinguish neutron population and gamma rays originated in plasma focus and SNM and evaluate the reactivity and other neutronic properties of the SNM [55, 56]. As far as the SNM target is concerned, it is made from fissile material, usually highly enriched uranium, or plutonium. The SNM target could be represented by static or dynamic device which implodes by an act of detonation of standard explosive (e.g. TNT). An example of the plasma focus static NDSE tests performed by the Los Alamos National Laboratory (LANL) at the Nevada National Security Site (NNSS) is presented in [55] and [56]. For illustration, see fig. 2.8 [52]. In these experiments, two kinds of SNM targets based on highly enriched uranium encapsulated in aluminum and polyethylene were tested. The SNM was placed in the downstream direction 3 m away of the Sodium device. The Sodium device is a 2-MA, 350-kJ plasma focus newly developed by National Security Technologies (NSTec). Since in the NDSE tests the neutron pulse should be as short as possible, unlike the classical plasmafocus, an additional cathode was placed on the z-axis in the distance of about 4 cm from the cathode [55]. By experiments and simulations, it was found that such a limited pinch region reduces the typical neutron pulse length by avoiding the formation of multiple pinches [55]. Using a deuterium-tritium gas mixture, the yield of DT neutrons with the energy of about 14 MeV exceeds 1012

per single pulse with the FWHM bellow 100 ns [55, 56]. The detailed description of the diagnostics system and experimental results is beyond the scope of this thesis.

Another very actual application of plasma foci is the flash neutron radiography with high spatial resolution and short exposure time. A notable development of the neutron flash radiography techniques using the Gemini device is presented in [59]. The Gemini device is a plasma focus developed by the NSTec with 1 MJ of the stored electrical energy and the maximum current of 3 MA (see fig. 2.9) [57, 58]. Using a deuterium gas filling, a yield of 2.45 MeV DD neutrons reaches the order of 1011. The neutron radiography image of the tungsten block was converted into the visible light by BC-400 plastic scintillatior. With a help of SMIX Ultra High Speed Framing Camera [61], a sequence of 16 neutron radiography images of a tungsten block was acquired [59]. The exposure time of the individual images was 5 ns and the images were separated by intervals of 10 ns [59]. Such a flash neutron radiography technique seems to be promised for probing of high-density dynamic systems, for example implosions of uranium [60]. Moreover, in the plasma focus NDSE tests, the probing neutrons can be produced by the same plasma focus used to cause the

Figure 2.9: The Gemini plasma focus: stored electrical energy of 1 MJ, current pulse maximum of 3 MA, DD neutron yield above 1012 _{per single shot [57].}

fission [59].